1. Field of the Invention
The present invention is directed to a tesseral gradient coil for use in a nuclear magnetic resonance tomography apparatus of the type having a fundamental field magnet system with a hollow-cylindrical interior having a central axis in the z-direction of a Cartesian x-y-z coordinate system.
2. Description of the Prior Art
Gradient coils are known in the art for use in nuclear magnetic resonance tomography devices, the coils being disposed in the hollow-cylindrical interior of the fundamental field magnet system of the apparatus. It is known to provide a tesseral gradient coil which includes two segments proceeding in the azimuthal direction and conductors connecting these segments. Such a gradient coil is. described, for example, in European application 0 073 402 corresponding to U.S. Pat. No. 4,486,711. This known gradient coil consists of a plurality of saddle-shaped individual coils arranged on a cylindrical carrier, the longitudinal axis of the cylindrical carrier usually being referred to as the z-direction. Three gradient directions are required for nuclear magnetic resonance tomography, namely in the x-direction, y-direction and z-direction of a Cartesian coordinate system. Those gradient coils which generate magnetic field gradients in a direction perpendicular to the longitudinal cylinder axis, i.e. in the x-direction and the y-direction, are referred to as tesseral gradient coils.
A gradient coil system is disclosed in European application 0 216 590 wherein saddle coils which generate the useful field for the gradients are arranged on a first imaginary cylindrical surface. Further saddle coils which compensate the field of the gradient coils in the outward direction are arranged on a second imaginary cylindrical surface having a larger diameter. The currents induced in the cold shield of the fundamental field magnet caused by a field change of the gradient coils are thus considerably reduced. These currents are disturbing, particularly given gradients which are rapidly switched.
Gradients which are rapidly switched are needed, in particular, in a method of operating a nuclear magnetic resonance tomography apparatus known as the echo planar method. After a single excitation, a gradient field is reversed in polarity until the measured data for a complete slice of the examination subject are obtained. Because this must occur before the nuclear magnetic resonance signal has decayed, short switching times are necessary. The fast flux change dB/dt required for this purpose induces currents not only in the cold shield of the fundamental field magnet system which surrounds the gradient coils, but also induces currents in the examination subject. In the extreme case, the currents induced in this manner may stimulate a patient's nerves and may be physiologically unpleasant.
In conventional tesseral gradient coils, the useful field, functioning to generate magnetic field gradients, is lower than a parasitic magnetic field which is unavoidably also generated, so that the induced currents essentially derive from the parasitic magnetic field.